Pile drivers are used in the construction industry to drive piles, also known as posts, into the ground. Piles are used to support massive structures such as bridges, towers, dams and skyscrapers. Piles, or posts, may be made of timber, steel, concrete or composites. Driving a pile into the ground requires high impact energy to overcome soil resistance. However, the impact energy must not be so large as to damage the pile during installation.
Impact stresses are directly related to the impact energy delivered to the pile by a driving element such as a hammer. During impact, the energy transferred to the pile is a function of force, F(t), and velocity, v(t), both of which vary in time. The impact energy as a function of time, E(t), is calculated as follows:E(t)=∫F(t)v(t)dt.
The impact energy may be approximated to be the kinetic energy of a pile driving hammer just before it impacts the pile head, i.e., E=½ mv2. However, not all of this kinetic energy is transferred to the pile because of the inelasticity of the collision, which results in deformation and energy dissipation in the form of heat and sound.
There are a variety of pile driving machines currently known in the industry. There are simple drop-hammer pile drivers that use a cable, winch and crane to raise a mass above the pile and simply let the hammer free-fall onto the top of the pile (also known as the pile head), as illustrated in U.S. Pat. No. 4,660,655 (Wilner). Sometimes the drop hammer has a vertical guide or rail to ensure greater accuracy during the drop. These guided drop hammers are shown in U.S. Pat. No. 5,978,749 (Likins, Jr. et al.) and in U.S. Pat. No. 6,301,551 (Piscalko et al.). Pile drivers may also be hydraulically actuated as in U.S. Pat. No. 5,090,485 (Pomonik et al.) or pneumatically driven as in U.S. Pat. No. 4,508,181 (Jenne). There are also diesel-powered pile drivers (which are also known as free piston internal combustion pile drivers). The diesel pile driver uses the piston as the impacting hammer. This type of pile driver is described in U.S. Pat. No. 5,727,639 (Jeter).
One of the main recurrent problems in pile driving is controlling the impact of the hammer on the pile. If the impact energy is too little, the pile does not penetrate the soil and time and energy is lost. If the impact energy is too great, the pile may be damaged or broken. Indeed, concrete piles are susceptible to cracking if the impact stresses are too large.
Traditionally, foundation engineers have relied on static or dynamic analyses, probe piles and static testing to ensure a safe and efficient installation. However, the dynamic formulae are intrinsically inaccurate because the dynamic modeling of the hammer, driving system, pile and soil is based on simplifications and assumptions that do not always simulate reality. Even if dynamic models were further refined, they would still not be able to account for the fact that soil conditions may vary with depth or may change due to repetitive impacting. Recent attention has been paid to the question of measuring the impact energy transferred from the hammer to the pile. In U.S. Pat. No. 5,978,749, Likins Jr. discloses a system for recording data from sensors. The impact energy for the subsequent impact is then manually adjusted, for example, by varying the drop height of the drop-hammer pile driver or by throttling the diesel pile driver to vary the ram stroke. Likewise, in U.S. Pat. No. 6,301,551 (Piscalko et al.), a pile driver analyzer (PDA) collects data from sensors located on the pile itself.
However, certain drawbacks are evident from the prior art designs. The manual control of impact energy is both time-consuming and inaccurate. The types of parameters that can be used to control pile driving also tend to be limited.
Accordingly, an improved means of controlling the impact energy of the hammer in a pile driver is needed.